Coal liquefaction is a general term referring to a family of processes for producing liquid fuels from coal. Specific liquefaction technologies generally fall into two categories: direct (DCL) and indirect liquefaction (ICL) processes. Indirect liquefaction processes generally involve gasification of coal to a mixture of carbon monoxide and hydrogen (syngas) and then using a process such as Fischer-Tropsch to convert the syngas mixture into liquid hydrocarbons. By contrast, direct liquefaction processes convert coal into liquids directly, without the intermediate step of gasification, by breaking down its organic structure with application of solvents or catalysts in a high pressure and temperature environment. Since liquid hydrocarbons generally have a higher hydrogen-carbon molar ratio than coals, either hydrogenation or carbon-rejection processes must be employed in both ICL and DCL technologies.
As coal liquefaction generally is a high-temperature/high-pressure process, it requires a significant energy consumption and, at industrial scales (thousands of barrels/day), multi-billion dollar capital investments. Thus, coal liquefaction is only economically viable at historically high oil prices, and therefore presents a high investment risk.
One of the main methods of direct conversion of coal to liquids by hydrogenation process is the Bergius process, developed by Friedrich Bergius in 1913. In this process, dry coal is mixed with heavy oil recycled from the process. Catalyst is typically added to the mixture. The reaction occurs at between 400 °C (752 °F) to 500 °C (932 °F) and 20 to 70 MPa hydrogen pressure. The reaction can be summarized as follows:
- n C + (n + 1) H2 → CnH2 n + 2
After World War I several plants were built in Germany; these plants were extensively used during World War II to supply Germany with fuel and lubricants. The Kohleoel Process, developed in Germany by Ruhrkohle and VEBA, was used in the demonstration plant with the capacity of 200 ton of lignite per day, built in Bottrop, Germany. This plant operated from 1981 to 1987. In this process, coal is mixed with a recycle solvent and iron catalyst. After preheating and pressurizing, H2 is added. The process takes place in a tubular reactor at the pressure of 300 bar and at the temperature of 470 °C (880 °F). This process was also explored by SASOL in South Africa.
In 1970-1980s, Japanese companies Nippon Kokan, Sumitomo Metal Industries and Mitsubishi Heavy Industries developed the NEDOL process. In this process, coal is mixed with a recycled solvent and a synthetic iron-based catalyst; after preheating, H2 is added. The reaction takes place in a tubular reactor at temperature between 430 °C (810 °F) and 465 °C (870 °F) at the pressure 150-200 bar. The produced oil has low quality and requires intensive upgrading. H-Coal process, developed by Hydrocarbon Research, Inc., in 1963, mixes pulverized coal with recycled liquids, hydrogen and catalyst in the ebullated bed reactor. Advantages of this process are that dissolution and oil upgrading are taking place in the single reactor, products have high H/C ratio, and a fast reaction time, while the main disadvantages are high gas yield (this is basically a thermal cracking process), high hydrogen consumption, and limitation of oil usage only as a boiler oil because of impurities.
The SRC-I and SRC-II (Solvent Refined Coal) processes developed by Gulf Oil and implemented as pilot plants in the United States in the 1960s and 1970s. The Nuclear Utility Services Corporation developed hydrogenation process which was patented by Wilburn C. Schroeder in 1976. The process involved dried, pulverized coal mixed with roughly 1wt% molybdenum catalysts. Hydrogenation occurred by use of high temperature and pressure synthesis gas produced in a separate gasifier. The process ultimately yielded a synthetic crude product, Naphtha, a limited amount of C3/C4 gas, light-medium weight liquids (C5-C10) suitable for use as fuels, small amounts of NH3 and significant amounts of CO2. Other single-stage hydrogenation processes are the Exxon Donor Solvent Process, the Imhausen High-pressure Process, and the Conoco Zinc Chloride Process.
There are also a number of two-stage direct liquefaction processes; however, after 1980s only the Catalytic Two-stage Liquefaction Process, modified from the H-Coal Process; the Liquid Solvent Extraction Process by British Coal; and the Brown Coal Liquefaction Process of Japan have been developed.
Shenhua, a Chinese coal mining company, decided in 2002 to build a direct liquefaction plant in Inner Mongolia, with barrel capacity of 20 thousand barrels per day (3.2×103 m3/d). First tests were implemented at the end of 2008. A second and longer test campaign was started in October 2009.
Chevron Corporation developed a process invented by Joel W. Rosenthal called the Chevron Coal Liquefaction Process (CCLP). It is unique due the close-coupling of the non-catalytic dissolver and the catalytic hydroprocessing unit. The oil produced had properties that were unique when compared to other coal oils; it was lighter and had far fewer heteroatom impurities. The process was scaled-up to the 6 ton per day level, but not proven commercially.
Pyrolysis and carbonization processes
A number of carbonization processes exist. The carbonization conversion occurs through pyrolysis or destructive distillation, and it produces condensable coal tar, oil and water vapor, non-condensable synthetic gas, and a solid residue-char. The coal tar and oil are then further processed by hydrotreating to remove sulfur and nitrogen species, after which they are processed into fuels.
The typical example of carbonization is the Karrick process. In this low-temperature carbonization process, coal is heated at 680 °F (360 °C) to 1,380 °F (750 °C) in the absence of air. These temperatures optimize the production of coal tars richer in lighter hydrocarbons than normal coal tar. However, the produced liquids are mostly a by-product and the main product is semi-coke, a solid and smokeless fuel.
The COED Process, developed by FMC Corporation, uses a fluidized bed for processing, in combination with increasing temperature, through four stages of pyrolysis. Heat is transferred by hot gases produced by combustion of part of the produced char. A modification of this process, the COGAS Process, involves the addition of gasification of char. The TOSCOAL Process, an analogue to the TOSCO II oil shale retorting process and Lurgi-Ruhrgas process, which is also used for the shale oil extraction, uses hot recycled solids for the heat transfer.
Liquid yields of pyrolysis and Karrick processes are generally low for practical use for synthetic liquid fuel production. Furthermore, the resulting liquids are of low quality and require further treatment before they can be used as motor fuels. In summary, there is little possibility that this process will yield economically viable volumes of liquid fuel.
Indirect conversion processes
Indirect coal liquefaction (ICL) processes operate in two stages. In the first stage, coal is converted into syngas (a purified mixture of CO and H2 gas). In the second stage, the syngas is converted into light hydrocarbons using one of three main processes: Fischer-Tropsch synthesis, Methanol synthesis with subsequent conversion to gasoline or petrochemicals, and methanation.
Fischer-Tropsch is the oldest of the ICL processes. It was first used on large technical scale in Germany between 1934 and 1945 and is currently being used by Sasol in South Africa.
In methanol synthesis processes syngas is converted to methanol, which is subsequently polymerized into alkanes over a zeolite catalyst. This process, under the moniker MTG (MTG for "Methanol To Gasoline"), was developed by Mobil in early 1970s, and is being tested at a demonstration plant by Jincheng Anthracite Mining Group (JAMG) in Shanxi, China.
Methanation reaction converts syngas to synthetic natural gas (SNG). The Great Plains Gasification Plant in Beulah, North Dakota is a coal-to-SNG facility producing 160 million cubic feet per day of SNG, and has been in operation since 1984. 
Most coal liquefaction processes are associated with significant CO2 emissions, resulting either from the gasification process or from generation of heat and electricity that serve as energy inputs to the reactors. High water consumption in water-gas shift or methane steam reforming reactions is another adverse environmental effect. On the other hand, synthetic fuels produced by coal liquefaction processes tend to be 'cleaner' than naturally occurring crudes, as heteroatom (e.g. sulfur) compounds are not synthesized or are excluded from the final product.
Pyrolysis of coal produces polycyclic aromatic hydrocarbons, which are known carcinogens. There are also considerations with respect to global warming, especially if coal liquefaction is conducted without carbon capture and storage technologies.
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